Summary The reason why women sustain more soft tissue injury than men during physical activity is unknown. Connective tissue properties and extracellular matrix adaptability in human tendon were investigated in models that addressed biochemical, physiological and biomechanical aspects of tendon connective tissue in response to mechanical loading. Habitual training resulted in a larger patellar tendon in men but not in women. Following an acute bout of exercise, men had an elevated tendon collagen synthesis rate and this effect was less pronounced or absent in women. Moreover, levels of circulating oestrogen affected the acute exercise‐related increase in collagen synthesis. Finally, the mechanical strength of isolated tendon collagen fascicles in men surpassed that of women. Thus, compared to men, women have (i) an attenuated tendon hypertrophy response to habitual training; (ii) a lower tendon collagen synthesis rate following acute exercise; (iii) a rate of tendon collagen synthesis which is further attenuated with elevated estradiol levels; and (iv) a lower mechanical strength of their tendons. These data indicate that tendons in women have a lower rate of new connective tissue formation, respond less to mechanical loading, and have a lower mechanical strength, which may leave the tissue more susceptible to injury.

The extracellular matrix (ECM) of connective tissues enables linking to other tissues, and plays a key role in force transmission and tissue structure maintenance in tendons, ligaments, bone and muscle. ECM turnover is influenced by physical activity, and both collagen synthesis and metalloprotease activity increase with mechanical loading. This can be shown by determining propeptide and proteinase activity by microdialysis, as well as by verifying the incorporation of infused stable isotope amino acids in biopsies. Local tissue expression and release of growth factors for ECM such as IGF-1, TGF-beta and IL-6 is enhanced following exercise. For tendons, metabolic activity (e.g. detected by positron emission tomography scanning), circulatory responses (e.g. as measured by near-infrared spectroscopy and dye dilution) and collagen turnover are markedly increased after exercise. Tendon blood flow is regulated by cyclooxygenase-2 (COX-2)-mediated pathways, and glucose uptake is regulated by specific pathways in tendons that differ from those in skeletal muscle. Chronic loading in the form of physical training leads both to increased collagen turnover as well as to some degree of net collagen synthesis. These changes modify the mechanical properties and the viscoelastic characteristics of the tissue, decrease its stress-susceptibility and probably make it more load-resistant. The mechanical properties of tendon fascicles vary within a given human tendon, and even show gender differences. The latter is supported by findings of gender-related differences in the activation of collagen synthesis with exercise. These findings may provide the basis for understanding tissue overloading and injury in both tendons and skeletal muscle.

The present study investigated the mechanical properties of tendon fascicles from the anterior and posterior human patellar tendon. Collagen fascicles from the anterior and posterior human patellar tendon in healthy young men (mean +/- SD, 29.0 +/- 4.6 yr, n = 6) were tested in a mechanical rig. A stereoscopic microscope equipped with a digital camera recorded elongation. The fascicles were preconditioned five cycles before the failure test based on pilot data on rat tendon fascicle. Human fascicle length increased with repeated cycles (P < 0.05); cycle 5 differed from cycle 1 (P < 0.05), but not cycles 2-4. Peak stress and yield stress were greater for anterior (76.0 +/- 9.5 and 56.6 +/- 10.4 MPa, respectively) than posterior fascicles (38.5 +/- 3.9 and 31.6 +/- 2.9 MPa, respectively), P < 0.05, while yield strain was similar (anterior 6.8 +/- 1.0%, posterior 8.7 +/- 1.4%). Tangent modulus was greater for the anterior (1,231 +/- 188 MPa) than the posterior (583 +/- 122 MPa) fascicles, P < 0.05. In conclusion, tendon fascicles from the anterior portion of the human patellar tendon in young men displayed considerably greater peak and yield stress and tangent modulus compared with the posterior portion of the tendon, indicating region-specific material properties.

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Lower strength of the human posterior patellar tendon seems unrelated to mature collagen cross-linking and fibril morphology. J Appl Physiol 108: 47-52, 2010. First published November 5, 2009 doi:10.1152/japplphysiol.00944.2009The human patellar tendon is frequently affected by tendinopathy, but the etiology of the condition is not established, although differential loading of the anterior and posterior tendon may be associated with the condition. We hypothesized that changes in fibril morphology and collagen cross-linking would parallel differences in material strength between the anterior and posterior tendon. Tendon fascicles were obtained from elective ACL surgery patients and tested micromechanically. Transmission electron microscopy was used to assess fibril morphology, and collagen cross-linking was determined by HPLC and calorimetry. Anterior fascicles were markedly stronger (peak stress: 54.3 Ϯ 21.2 vs. 39.7 Ϯ 21.3 MPa; P Ͻ 0.05) and stiffer (624 Ϯ 232 vs. 362 Ϯ 170 MPa; P Ͻ 0.01) than posterior fascicles. Notably, mature pyridinium type cross-links were less abundant in anterior fascicles (hydroxylysylpyridinoline: 0.859 Ϯ 0.197 vs. 1.416 Ϯ 0.250 mol/mol, P ϭ 0.001; lysylpyridinoline: 0.023 Ϯ 0.006 vs. 0.035 Ϯ 0.006 mol/mol, P Ͻ 0.01), whereas pentosidine and pyrrole concentrations showed no regional differences. Fibril diameters tended to be larger in anterior fascicles (7.819 Ϯ 2.168 vs. 4.897 Ϯ 1.434 nm 2 ; P ϭ 0.10). Material properties did not appear closely related to cross-linking or fibril morphology. These findings suggest region-specific differences in mechanical, structural, and biochemical properties of the human patellar tendon.